A wireless telecommunications system has been proposed in which a geographical area is divided into cells, each cell having one or more central terminals (CTs) for communicating over wireless links with a number of subscriber terminals (STs) in the cell. These wireless links are established over predetermined frequency channels, a frequency channel typically consisting of one frequency for uplink signals from a subscriber terminal to the central terminal, and another frequency for downlink signals from the central terminal to the subscriber terminal.
The system finds a wide variety of possible applications, for example in rural, remote, or sparsely populated areas where the cost of laying permanent wire or optical networks would be too expensive, in heavily builtup areas where conventional wired systems are at full capacity or the cost of laying such systems would involve too much interruption to the existing infrastructure or be too expensive, and so on.
The central terminal is typically connected to a telephone network and exists to relay messages from subscriber terminals in the cell controlled by the central terminal to the telephone network, and vice versa. By this approach, an item of telecommunications equipment connected to a subscriber terminal may make an outgoing call to the telephone network, and may receive incoming calls from the telephone network.
Due to bandwidth constraints, it is not practical for each individual subscriber terminal to have its own dedicated frequency channel for communicating with a central terminal. Hence, techniques have been developed to enable data items relating to different wireless links (i.e. different ST-CT communications) to be transmitted simultaneously on the same frequency channel without interfering with each other. One such technique involves the use of a "Code Division Multiple Access" (CDMA) technique whereby a set of orthogonal codes may be applied to the data to be transmitted on a particular frequency channel, data items relating to different wireless links being combined with different orthogonal codes from the set. Signals to which an orthogonal code has been applied can be considered as being transmitted over a corresponding orthogonal channel within a particular frequency channel.
One way of operating such a wireless telecommunications system is in a fixed assignment mode, where a particular ST is directly associated with a particular orthogonal channel of a particular frequency channel. Calls to and from items of telecommunications equipment connected to that ST will always be handled by that orthogonal channel on that particular frequency channel, the orthogonal channel always being available and dedicated to that particular ST.
However, as the number of users of telecommunications networks increases, so there is an ever increasing demand for such networks to be able to support more users. To increase the number of users that may be supported by a single central terminal, an alternative way of operating such a wireless telecommunications system is in a Demand Assignment mode, in which a larger number of STs are associated with the central terminal than the number of traffic-bearing orthogonal channels available. These orthogonal channels are then assigned to particular STs on demand as needed. This approach means that far more STs can be supported by a single central terminal than is possible in a fixed assignment mode, the exact number supported depending on the level of dial tone service that the service provider desires.
In such a Demand Assignment system, it is clear that when a wireless link is to be established between a particular subscriber terminal and the central terminal, then the actual radio slot assigned for that wireless link will depend on the current usage of other radio slots in the wireless telecommunications system, and so the actual radio slot assigned to a particular subscriber terminal for each subsequent call is likely to vary on a call-by-call basis. For the purposes of the present application, the term "radio slot" refers to the bandwidth elements into which each frequency channel is sub-divided, these radio slots being assigned to particular calls as required. Hence, for the above-discussed CDMA technique, the radio slots may be the orthogonal channels provided within the frequency channel, or alternatively may be a predetermined portion of an orthogonal channel, assuming techniques are employed to enable data items pertaining to more than one wireless link to be transmitted within a single orthogonal channel of a particular frequency channel.
UK patent application no. 9626566.5 describes one such technique whereby selected orthogonal channels may be subdivided to form additional orthogonal channels. Further, UK patent application no. 9626567.3 describes an alternative approach in which selected orthogonal channels may be sub-divided in the time dimension by employing TDM techniques.
Given that, in a Demand Assignment system as discussed above, the actual radio slot assigned for a wireless link between a particular subscriber terminal and the central terminal may vary on a call-by-call basis, it is clear that one problem that arises is how to ensure that both the central terminal and the subscriber terminal establish the wireless link on the correct radio slot each time a call is to take place.